Check your brain

: : It means you are about to witness or be involved in something so stupid that you can only enjoy it if you seemingly have no brain. It's similar to checking your hat and/or coat as you enter a theater or fancy restaurant.

The phrase also applies to "entering" a belief system that makes so little sense - in the speaker's or writer's opinion, of course - that to subscribe to it, you have to suppress your power of thought. ~rb